Most present day systems require a dedicated electrical contact in order to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each power supply being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter inductor in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers having a tight coupling between a primary transmitter inductor and a secondary receiver coil. By separating the primary transmitter inductor and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to, or on top of, the transmitter inductor in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer approach known as the Qi Specifications has been defined and is currently being developed further. This approach allows power transmitter devices that meet the Qi Specifications to be used with power receiver devices that also meet the Qi Specifications without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi Specification is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Specification documents can be found.
Many wireless power transmission systems, such as e.g. Qi, supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information to the power transmitter that may allow this to adapt to the specific power receiver or the specific conditions experienced by the power receiver.
In many systems, such communication is by load modulation of the power transfer signal. Specifically, the communication is achieved by the power receiver performing load modulation wherein a load applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current of the transmitter inductor) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which can be detected by a change in the amplitude and/or phase of the transmitter inductor current or voltage.
More information of the application of load modulation in Qi can e.g. be found in chapter 6 of part 1 of the Qi wireless power specification (version 1.0).
Wireless power transmitters constructed according to the Qi v1.1 specification operate in the so-called inductive regime. In this mode, power transfer occurs at tight coupling (coupling factor typically above 0.3) with relatively high efficiency. If a larger distance (“Z-distance”) or more positioning freedom of the receiver is desired, power transfer typically occurs in the so-called resonant regime with loose coupling (coupling factor typically below 0.3). In the resonant regime, the resonance frequencies of power transfer resonance circuits at the power transmitter and at the power receiver should match to achieve the maximum efficiency. However, with an increased distance between the resonance circuits, the load modulation communication from power receiver to power transmitter becomes increasingly difficult. In resonant mode, the power transmitter's resonant circuit typically becomes underdamped which makes it sensitive to intermodulation distortion (with the intermodulation being between the resonance frequency of the power transmitter and the drive frequency). Indeed, if the resonance frequency and the drive frequency of the power transmitter do not match, intermodulation frequencies appear, resulting in a degradation in communication performance, and often making the demodulation process at the power transmitter problematic or even impossible.
In order to address the intermodulation problems, it has been proposed to employ a tuneable resonance circuit at the power transmitter, i.e. it has been proposed to use a resonance circuit for which the resonance frequency can be dynamically varied. In such a system, the drive frequency and the resonance frequency of the power transmitter may both be adapted to be the same as the resonance frequency of the power receiver. This may ensure that the system efficiently operates in the resonance mode while at the same time preventing (or at least mitigating) intermodulation effects between the drive frequency and the power transmitter resonance circuit. It may further in many scenarios allow the system to adapt and compensate for variations and tolerances of component values etc. An example of a system setting the frequencies of the drive signal, the transmitter resonance frequency and the receiver resonance frequency to the same value is provided in US20040130915A1.
However, it has been found that the approach also introduces some issues. Firstly, in some systems, the transmitted power level is adapted by changing the drive frequency (i.e. power control may be implemented using adjustments of the drive frequency). However, if the frequencies are locked together, this is no longer a feasible option. Instead, the power level is typically controlled by the amplitude or duty cycle of the drive signal. Furthermore, it has been found that communication performance is still not optimal, and indeed that it may in some scenarios be significantly degraded, and potentially reliable communication performance may not be possible.
An improved power transfer approach would accordingly be advantageous. In particular, an approach that allows improved operation, improved power transfer, increased flexibility, facilitated implementation, facilitated operation, improved communication, reduced communication errors, improved power control, improved power transfer, and/or improved performance would be advantageous.